Electrolytics "blow up"

[Steve Harrison]

At 11:02 AM 1999-11-28 -0500, mike bryce wrote:
>bill... sounds like you're headed in the right direction... here's some more
>tips for you in your quest for QRM.
>
>
>There is a RF choke and a .01 (.001?) cap at the + side of the 40 mf cap. If
>you lift the RF choke, you have in fact taken the plate voltage away from
>the final. Keeps those drive pots from smoking!!

DON'T REMOVE PLATE VOLTAGE WITHOUT ALSO REMOVING SCREEN VOLTAGE!!
Otherwise, the screen acts like the "missing" plate and draws as much
current as any dropping resistors in series with it allow. And there goes
Tube.

>Check the other resistors in the voltage dividers in the power supply
>section. There's about four of them if I recall. One may be changed to a
>higher value, and throw the dividers out to lunch.

I've been building a lot of stuff recently, being unemployed and having
lots of time on my hands (like, 24/7...). So I've been using up my resistor
stock in a really prodigious manner, including the carbon comps. Since I
like to know the resistance of things that are supposed to be resistors,
I've gotten into the habit of measuring things before installing them. And
without fail, NONE of my carbon composition resistors are anywhere close to
what they should be. Usually, they're high; but those that have been
overheated by too much soldering (I don't keep those that have been
overheated in a circuit) sometimes measure way too low, as much as 50%.
Carbon film types, OTOH, are almost always within spec. If you've got a
circuit with carbon comps whose value can be important such as a voltage
divider, MEASURE THEM.

>If you don't have a variac, use a 60 watt light bulb in series with the 110
>vac primary. Saves a lot of pieces parts...

If you do have a variac, USE A FUSE IN THE INPUT OR OUTPUT!! Saves your
variac.... And if, when powering something, rotating the variac's knob
feels a little different such as kind of like a hum as you rotate it,
BEWARE... you may be drawing lots of current. If you can find one, put a 1
ohm, 10 watt resistor in series with the output and measure the voltage
across it with your old Tandy VOM on AC-Volts to keep an eye on the current
into your BA as you turn up the juice.

>Since we're not lunching missles, don't worry about getting a 40 mf. just
>about anything close will work. i.e. a 47mf, 56 mf. as long as the working
>voltage is equal to or even higher will do.

Even a 20 mF will probably work although allow a lil bit of hum. And an 80
probably won't be bad, either. BUT ONLY IN POWER SUPPLY CIRCUITS, and only
where not in series with other electrolytics!! If the electrolytic is being
used as a bypass in the cathode of an audio amp, for example, it can
drastically change the audio frequency response of the circuit. If there's
several in series, they all need to be roughly the same value capacitance,
within, say, 50% as Mike suggests.

73, Steve K0XP

Sponsored by the City of Tempe 

Listserver Submissions:  heath at listserv.tempe.gov
Listserver Subscription: listserv at listserv.tempe.gov - "subscribe heath 'name' 'call'"
Listserver Unsubscribe: listserv at listserv.tempe.gov - -"signoff heath"